Method of driving a well pump

ABSTRACT

Production liquid from a well is pumped to the surface by a hydraulically driven reciprocating piston pump. The pump is driven in a drive stroke by a flow of hydraulic fluid supplied through a single tubing line applied to a drive side of a drive piston and in a return stroke by the weight of the production liquid heavier than the hydraulic fluid applied to the return side of the drive piston. The production liquid is pumped though the well casing. A control valve causes application of pressure in the hydraulic fluid for the drive stroke and release for the return stroke. The pump body includes a first duct communicating between the tubing and the bottom drive side of the drive piston and a second duct communicating between the well casing and the top drive return of the drive piston.

This invention relates to a method of driving a well pump.

BACKGROUND OF THE INVENTION

Down-hole hydraulic pumps with the valving, piston and pump (and itsvariations) were originally developed under the trade names “Kobe” and“Oilmaster”. Both have been available to the industry for more than fivedecades. The product enjoys worldwide acceptance under the currentdirection of Weatherford Oil Tool. These pumps find special applicationlifting large volumes of light oil in deep wells.

More recently Canadian application 2,258,237 by Cunningham suggestedbringing the valving to the surface, and proposed using a downholedouble acting hydraulic piston, three (3) strings of tube and aconventional oil well pump for placement in a horizontally drilled heavyoil well. The double acting feature of the hydraulic piston would beparticularly useful as a pump pull-down in the highly viscous heavy oilapplications for which the system was conceived.

Canadian application 2,260,518 proposes using a down-hole rotaryhydraulic drive, coupled to a progressing cavity pump rather than thereciprocating version suggested by the Cunningham application. Bothaddress the task of pumping heavy oil in deviated well-bores.

U.S. Pat. No. 7,380,608 (Geier) issued Jun. 3, 2008 discloses ahydraulic pump which uses hydraulic pressure from the surface to drivethe reciprocating pump where a down hole counterbalance chambergenerates a back pressure from the hydraulic pressure which is used todrive the pump back when the hydraulic pressure is released.

The disadvantages of an insert pumping system are widely acknowledged inthe oil and gas industry. Up front capital costs to install a pump jack,rod string and insert pump are significant and require the use of aworkover rig. Day rates and availability of workover rigs in periods ofhigh activity warrant special consideration but generally are viewed asa necessary evil and are accepted as a cost of doing business. Onceinstalled, this pumping system requires regular maintenance,mobilization of a workover rig is again necessary in addition to thecosts of replacing tubing, rods and the BHP during the workover. InNorthern areas the costs of a simple pump change can exceed $100,000.00.Due to the above factors a review of existing systems was conducted inan effort to identify a cost effective alternative to a rod pumpingsystem.

SUMMARY OF THE INVENTION

It is one object of the invention to provide a method of pumping liquidfrom a production location of a down hole well to a surface location.

According to one aspect of the invention there is provided a method ofpumping liquid from a production location of a down hole well to asurface location,

where the well includes a well casing extending from the surfacelocation to the production location

the method comprising:

providing a hydraulic pump at the surface location for generating a flowof hydraulic fluid under pressure;

providing a tubing extending from the pump through the well casing tothe production location, the tubing being connected so as to carry thehydraulic fluid under pressure through the tubing to the productionlocation;

operating a control valve to cause application of pressure in thehydraulic fluid to be applied into the fluid in the tubing and to causethe pressure to be released;

locating the tubing in the well casing so that the well casing surroundsthe tubing so as to define a duct around the tubing for transport ofproduction liquid through the well casing;

providing at the production location a reciprocating piston pump havinga pump cylinder with a pump piston movable therein, an inlet forproduction liquid and an outlet for the production liquid communicatingwith the pump cylinder such that in inlet stroke of the pump pistoncauses a volume of the production liquid to be drawn from the inlet intothe cylinder and an outlet stroke of the pump piston causes the volumeto be expelled from the outlet;

locating the reciprocating piston pump in the well casing at theproduction location;

providing a packing between the reciprocating piston pump and the wellcasing such that the inlet is located below the packing and the outletis located above the packing such that the stroke of the pump pistoncarries the volume of liquid from the production location below thepacking to the well casing above the packing;

the reciprocating piston pump having a drive piston mounted in a drivecylinder and arranged such that a drive stroke of the drive piston inthe drive cylinder acts to cause the pump stroke of the pump piston inthe pump cylinder;

connecting the tubing to the reciprocating piston pump and providing afirst duct in the reciprocating piston pump so as to transfer hydraulicfluid under pressure from the tubing to the drive cylinder at a positionthereon on a first side of the drive piston such that pressure in thehydraulic fluid is communicated to the drive piston to drive the drivepiston along the drive cylinder in the drive stroke;

providing a second duct in the reciprocating piston pump connecting thedrive cylinder on a second side of the drive piston to the well casingabove the packing so as to allow communication of production liquid intoand out of the drive cylinder on the second side of the drive piston;

arranging the pressure in the hydraulic fluid to drive the drive pistonthrough the drive stroke to overcome the pressure of the productionliquid against the second side of the drive piston and to overcome thepressure of the production liquid against the pump piston to carry thevolume of liquid from the production location below the packing to thewell casing above the packing;

and causing the drive piston to move through a return stroke byoperating the control valve to effect releasing the pressure in thehydraulic fluid so that pressure in the production liquid in the wellcasing applied to the piston on the second side the pressure in appliesa force to the drive piston.

The tubing can be coiled tubing or a conventional jointed tubing system.

Preferably the density of the hydraulic fluid is arranged to be lessthan that of the production liquid so that the column of productionliquid in the well casing has a weight which is greater than that of thehydraulic fluid in the tubing.

Preferably pressure from the production liquid below the packing isapplied to the pump piston from the inlet and added to the pressure fromthe production liquid on the drive piston so as to cause the drivepiston to move through the return stroke.

Preferably the first and second ducts are formed in a cylindrical bodyof the reciprocating piston pump.

Preferably the drive cylinder is located in the reciprocating pistonpump below the pump cylinder. However other configurations are possible.

Preferably the first duct extends to the bottom of the drive cylinder.

Preferably the inlet and outlet include one way valves arranged in therequired orientation to allow inlet and outlet as required.

Preferably the first duct is in open communication with the tubing suchthat the hydraulic fluid is free to transfer back and forth between thetubing and the first side of the drive cylinder and wherein the secondduct is in open communication with the well casing such that theproduction liquid is free to transfer back and forth between the wellcasing and the second side of the drive cylinder.

According to a second aspect of the invention there is provided a methodof pumping liquid from a production location of a down hole well to asurface location, the method comprising:

providing a hydraulic pump at the surface location for generating a flowof hydraulic fluid under pressure;

providing a drive conduit extending from the pump to the productionlocation, the tubing being connected so as to carry the hydraulic fluidunder pressure through the tubing to the production location;

providing a return conduit from the production location to the surfacefor transport of production liquid;

operating a control valve to cause application of pressure in thehydraulic fluid to be applied into the fluid in the drive conduit and tocause the pressure to be released;

providing at the production location a reciprocating piston pump havinga pump cylinder with a pump piston movable therein, an inlet forproduction liquid and an outlet for the production liquid communicatingwith the pump cylinder such that in inlet stroke of the pump pistoncauses a volume of the production liquid to be drawn from the inlet intothe cylinder and an outlet stroke of the pump piston causes the volumeto be expelled from the outlet;

locating the reciprocating piston pump at the production location;

arranging the reciprocating piston pump such that the stroke of the pumppiston carries the volume of liquid from the production location intothe return conduit;

the reciprocating piston pump having a drive piston mounted in a drivecylinder and arranged such that a drive stroke of the drive piston inthe drive cylinder acts to cause the pump stroke of the pump piston inthe pump cylinder;

connecting the drive conduit to the reciprocating piston pump andproviding a first duct in the reciprocating piston pump so as totransfer hydraulic fluid under pressure from the drive conduit to thedrive cylinder at a position thereon on a first side of the drive pistonsuch that pressure in the hydraulic fluid is communicated to the drivepiston to drive the drive piston along the drive cylinder in the drivestroke;

providing a second duct in the reciprocating piston pump connecting thedrive cylinder on a second side of the drive piston to the returnconduit so as to allow communication of production liquid into and outof the drive cylinder on the second side of the drive piston;

arranging the pressure in the hydraulic fluid to drive the drive pistonthrough the drive stroke to overcome the pressure of the productionliquid against the second side of the drive piston and to overcome thepressure of the production liquid against the pump piston to carry thevolume of liquid from the production location below the packing to thereturn conduit;

and causing the drive piston to move through a return stroke byoperating the control valve to effect releasing the pressure in thehydraulic fluid so that pressure in the production liquid in the returnconduit applied to the piston on the second side the pressure in appliesa force to the drive piston.

While the arrangement described particularly hereinafter is intended tobe used with a single tubing inside the well casing with the productionpassing through the well casing around the tubing, the same concept canbe used to de-water gas wells using this pump. In this case, the pump islocated inside of jointed production tubing with the production tubinghaving an inlet below the perforations.

In this case the power fluid is supplied down a small diameter jointedor coiled tubing string as described above inside the production tubing,produced water passes up the annulus between the string and theproduction tubing, gas then produced to surface between the productiontubing and casing. Thus there is formed a three conduit system definedby the inner tubing carrying the hydraulic fluid, a surrounding tubingcarrying the pumped water and the well casing surrounding the outertubing and carrying the produced gas. This pump would typically beinstalled offshore but could also be used on shore.

According to a third aspect of the invention there is provided areciprocating piston pump for pumping liquid from a production locationof a down hole well to a surface location,

where the well includes a well casing and a tubing inside the wellcasing both extending from the surface location to the productionlocation;

the reciprocating piston pump comprising:

a pump body having defined therein a pump cylinder and a drive cylinder;

a pump piston movable in the pump cylinder;

a drive piston movable in the drive cylinder and arranged to drive thepump piston such that a drive stroke of the drive piston in the drivecylinder acts to cause the pump stroke of the pump piston in the pumpcylinder;

an inlet for production liquid at and an outlet for the productionliquid communicating with the pump cylinder such that in inlet stroke ofthe pump piston causes a volume of the production liquid to be drawnfrom the inlet into the cylinder and an outlet stroke of the pump pistoncauses the volume to be expelled from the outlet;

the reciprocating piston pump being arranged to be mounted in the wellcasing at the production location with a packing between thereciprocating piston pump and the well casing such that the inlet islocated below the packing and the outlet is located above the packingsuch that the stroke of the pump piston carries the volume of liquidfrom the production location below the packing to the well casing abovethe packing;

a coupling for connecting the tubing to the reciprocating piston pump;

a first duct in the pump body so as to transfer hydraulic fluid underpressure from the tubing to the drive cylinder at a position thereon ona first side of the drive piston such that pressure in the hydraulicfluid is communicated to the drive piston to drive the drive pistonalong the drive cylinder in the drive stroke;

and a second duct in the pump body connecting the drive cylinder on asecond side of the drive piston to the well casing above the packing soas to allow communication of production liquid into and out of the drivecylinder on the second side of the drive piston.

The arrangement herein is based on the concept that by setting a packerand supplying the hydraulic pressure through a single tubing in the wellcasing, two hydrostatic columns exist in the well bore. One of thecolumns can be used to produce the well and operate the intake stroke ofthe pump while the other column can be used in conjunction with ahydraulic pump at surface to operate the displacement stroke of thepump. The system may offer one or more of the following advantages:

1. The pump jack and rod string have been eliminated.

2. Tubing wear, rod string wear and stripping jobs have been eliminated.

3. A workover rig, trucking and associated personnel have beeneliminated.

4. The footprint now consists of a small prime mover (i.e., electricmotor), hydraulic pump and reservoir.

5. Live well operations can be safely carried out by the CT unit withsuperior well control at all times.

6. Reduced rig up and trip times with a CT unit.

7. Due to the utilization of differential areas and force multiplicationwithin the pump a low horsepower hydraulic surface pump will move thenearly hydrostatically balanced columns efficiently.

8. No exotic materials exist in the system and only requires off theshelf components readily available from the existing supply chain.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention will now be described in conjunctionwith the accompanying drawings in which:

FIG. 1 is a schematic illustration of the apparatus and method forpumping production liquid from a downhole well to the surface.

FIG. 2 is a schematic illustration of the downhole reciprocating pumpused in FIG. 1.

In the drawings like characters of reference indicate correspondingparts in the different figures.

DETAILED DESCRIPTION

The system illustrated comprises a down hole reciprocating piston pump10 which is mounted in a well bore 11 at a production location 12 whereproduction liquid 13 enters into the pump 10 for pumping to the surfacethrough a well casing 14. At the surface 15 above the ground 16 isprovided a liquid recovery system 17 of a conventional nature forreceiving the pumped liquid for transportation or storage as requiredusing conventional systems.

The pump is mounted within the well casing 14 by a packing system 18which locates the pump within the well casing and also divides wellcasing into a production zone below the packing and into atransportation zone above the packing. Thus, as is conventional in suchpumps, the liquid is transferred by the pump from the location below thepacking into the well casing so that the production liquid is carried upthe well casing by the addition of further liquid into the well casingby the pumping action.

In the surface equipment as shown in FIG. 1, there is provided ahydraulic pump 20 which generates pressure in a hydraulic fluid suppliedthrough a tubing 21. The tubing extends through the well casing to thepump 10 where the tubing is connected to the pump for supply of thehydraulic fluid under pressure to the pump.

The hydraulic pump 20 receives hydraulic fluid from a tank 22 which issupplied to the pump 20 through a filter 23. The pump 20 is driven by amotor 24 which can be of any suitable construction. The pump 20 suppliesthe hydraulic fluid under pressure to a relief valve 25 which releasesthe pressure back to the tank in the event that an over pressurecondition is encountered. A pressure control switch or actuator 26detects the pressure of the hydraulic fluid and operates a shuttle valve27 to release that pressure when required.

The tubing 21 can be a jointed tubing arrangement or can be formed bycoil tubing as is well known in the industry. The tubing carries thehydraulic fluid to the pump 10 and allows flow of the fluid in bothdirections depending upon the pressure applied. The pressure controlswitch 26 is operated to activate and release the pressure in the tubing21.

The tubing 21 constitutes the only supply to the downhole pump and areturn conduit for transfer of production liquid is defined by the wellcasing 14 in the annular area surrounding the tubing.

At the end of the tubing 21 immediately prior to the pump 10 is provideda release tool 28 which allows the pump to be released from the tubingas required. Arrangements of this type are well known. Immediately abovethe release tool 28 is provided a circulating valve 29 again of aconventional construction which allows the release from the tubing intothe well casing of required chemicals again in a manner which is wellknown to persons skilled in the art.

The pump 10 as best shown in FIG. 2 is therefore driven by theapplication of pressure in the hydraulic fluid and by the release ofthat pressure. The pump 10 comprises a pump body 30 formed as acylindrical body which is typically formed in a number of pieces orsections connected end to end using conventional constructions. Theindividual sections are then machined to provide the required ducts,bores and cylinders as described hereinafter. The arrangement andconstruction of these connected units is well known to a person skilledin the art so that description of the details is not necessary. Thepacking arrangement 18 is mounted on the cylindrical body and is againof a conventional construction well known for surrounding and mounting acylindrical pump of this type so that the pump is held in positionwithin the well bore.

The pump body 30 includes a pump cylinder 31 and a drive cylinder 32.Within the pump cylinder is provided a pump piston 33 which can movealong the cylinder in a pump stroke and a return stroke. In FIG. 2 thepiston is shown at the lower end of the cylinder 31 in an initialposition for commencement of a pumping stroke. The cylinder 31 has aninlet duct 35 with a check valve 36 allowing production liquid to enterform the formation 12 into the interior of the cylinder 31. The inlet 35is located below the packing 18 at a position as close as possible tothe top of the cylinder 31. The cylinder 31 also has an outlet 37 with acheck valve 38 allowing the production liquid form the cylinder 31 toescape in the pumping stroke into the interior of the well casing 14.Thus the pumping strokes of the pump piston 33 act to transfer thevolume of liquid from the cylinder so that the liquid is carried fromthe production formation 12 into the well casing for a pumping action.The return stroke of the piston 33 draws the liquid from the formation12 into the cylinder. The pumping stroke drives the liquid out throughthe outlet 37.

The pump piston 33 is connected by a piston rod 40 to a drive piston 41within the drive cylinder 32. The drive cylinder 32 is on the outer orlower end of the pump body beyond the pump cylinder. The drive cylinderand the pump cylinder are separated by suitable components of the bodyso that there is no transfer of liquid therebetween. A seal 42 isprovided on the piston rod 40 at the junction between the cylinders sothat the piston rod can slide through the seal.

The cylinder 32 connects at its first end with a duct 43 which extendslongitudinally of the body 30 in a longitudinal portion 44 through tothe upper end of the body where the duct 44 communicates with the tubing21. Thus the hydraulic fluid is free to communicate from the tubing 21through the duct 44 and inlet 43 into the first or lower end of thedrive cylinder 32. The upper end of the cylinder 32 connects with a ductinlet 45 which connects with a longitudinal duct 46 in the bodyextending longitudinally of the body to a position above the packing 18where the duct 45 extends outwardly of the body to define an opencommunication 47 with the production liquid within the well casing 14.Typically the ducts 44 and 46 are angularly offset around thecylindrical body so that they can be drilled along the body at thecommon diameter within the thickness of the body outside the cylinders.In the schematic illustration, these are shown for convenience ofillustration at the same angular position but spaced radially.

Both the first and second ducts communicating the hydraulic fluid andthe production liquid are free from valves so they provide a freecommunication of the liquids between the outside of the pump and theinterior of the cylinder 32 on opposite sides of the piston 41.

In operation, the control switch 26 is operated so as to activate thesupply of hydraulic fluid through the tubing 21 and the duct 44 to thecylinder 32 at the first end of the piston 41. This hydraulic fluid isarranged to be of a pressure which is sufficient to overcome the backpressure from the weight of the production liquid which is applied ontothe piston 33 by the column of liquid within the well casing. Thispressure is applied by the column of liquid passing through the outlet37 so that in order to provide a pumping stroke of the piston 33 thepiston 41 must overcome this pressure and carry the volume of liquidfrom the cylinder 31 through the outlet 37 into the well casing.

At the end of the pumping stroke, the control switch 26 is arranged todetect the increase in pressure in the hydraulic fluid which isgenerated by the piston 41 reaching the end of its stroke. Upon thisdetection of an increase in pressure, the switch 26 acts to release thepressure in the tubing 21 by releasing the valve 27 causing the fluid inthe tubing 21 to communicate back to the tank 22. As the pressure isreleased from the lower end of the cylinder 32, the back pressuregenerated by the column of production liquid within the well casingapplies a force onto the top of the piston 33 causing the piston tocommence a return stroke. As soon as the piston 33 passes the inlet 35,fluid commences to enter the cylinder from the formation 12 and thepressure in the formation 12 is also applied to the piston 33. These twopressures overcome the pressure against the bottom of the piston 41generated by the weight of the column of hydraulic fluid in the tubing21.

The production liquid has a density which is greater than the density ofthe hydraulic fluid. For this reason typically hydraulic fluid such asdiesel fuel can be selected which is relatively low in density so thatthere is a significant difference in the weight of the column ofproduction liquid relative to the weight of column of the hydraulicfluid. This extra weight of the column of production fluid applies thepressure to the upper surface of the piston ring 41 through the duct 46and 47.

Thus as the drive piston goes through its drive stroke and its returnstroke, the drive stroke hydraulic fluid is supplied through the tubing21 and the production liquid escapes from the cylinder to the duct 46.The fluids are free to flow in both directions. As soon as the hydraulicpressure is released, the column of production liquid generates apressure which is applied through the duct 46 to the top of the piston41 to move the piston through its return stroke. As soon as the piston33 passes the inlet 35 this backpressure from the production liquid isincreased by the addition of pressure from the formation 12 thus causingthe pistons to move through the return stroke.

Since various modifications can be made in my invention as herein abovedescribed, and many apparently widely different embodiments of same madewithin the spirit and scope of the claims without department from suchspirit and scope, it is intended that all matter contained in theaccompanying specification shall be interpreted as illustrative only andnot in a limiting sense.

1. A method of pumping liquid from a production location of a down holewell to a surface location, where the well includes a well casingextending from the surface location to the production location themethod comprising: providing a hydraulic pump at the surface locationfor generating a flow of hydraulic fluid under pressure; providing atubing extending from the pump through the well casing to the productionlocation, the tubing being connected so as to carry the hydraulic fluidunder pressure through the tubing to the production location; operatinga control valve to cause application of pressure in the hydraulic fluidto be applied into the fluid in the tubing and to cause the pressure tobe released; locating the tubing in the well casing so that the wellcasing surrounds the tubing so as to define a duct around the tubing fortransport of production liquid through the well casing; providing at theproduction location a reciprocating piston pump having a pump cylinderwith a pump piston movable therein, an inlet for production liquid andan outlet for the production liquid communicating with the pump cylindersuch that in inlet stroke of the pump piston causes a volume of theproduction liquid to be drawn from the inlet into the cylinder and anoutlet stroke of the pump piston causes the volume to be expelled fromthe outlet; locating the reciprocating piston pump in the well casing atthe production location; providing a packing between the reciprocatingpiston pump and the well casing such that the inlet is located below thepacking and the outlet is located above the packing such that the strokeof the pump piston carries the volume of liquid from the productionlocation below the packing to the well casing above the packing; thereciprocating piston pump having a drive piston mounted in a drivecylinder and arranged such that a drive stroke of the drive piston inthe drive cylinder acts to cause the pump stroke of the pump piston inthe pump cylinder; connecting the tubing to the reciprocating pistonpump and providing a first duct in the reciprocating piston pump so asto transfer hydraulic fluid under pressure from the tubing to the drivecylinder at a position thereon on a first side of the drive piston suchthat pressure in the hydraulic fluid is communicated to the drive pistonto drive the drive piston along the drive cylinder in the drive stroke;providing a second duct in the reciprocating piston pump connecting thedrive cylinder on a second side of the drive piston to the well casingabove the packing so as to allow communication of production liquid intoand out of the drive cylinder on the second side of the drive piston;arranging the pressure in the hydraulic fluid to drive the drive pistonthrough the drive stroke to overcome the pressure of the productionliquid against the second side of the drive piston and to overcome thepressure of the production liquid against the pump piston to carry thevolume of liquid from the production location below the packing to thewell casing above the packing; and causing the drive piston to movethrough a return stroke by operating the control valve to effectreleasing the pressure in the hydraulic fluid so that pressure in theproduction liquid in the well casing applied to the piston on the secondside the pressure in applies a force to the drive piston.
 2. The methodaccording to claim 1 wherein the density of the hydraulic fluid isarranged to be less than that of the production liquid so that thecolumn of production liquid in the well casing has a weight which isgreater than that of the hydraulic fluid in the tubing.
 3. The methodaccording to claim 1 wherein pressure from the production liquid belowthe packing is applied to the pump piston from the inlet and added tothe pressure from the production liquid on the drive piston so as tocause the drive piston to move through the return stroke.
 4. The methodaccording to claim 1 wherein the first and second ducts are formed in acylindrical body of the reciprocating piston pump.
 5. The methodaccording to claim 1 wherein the drive cylinder is located in thereciprocating piston pump below the pump cylinder.
 6. The methodaccording to claim 1 wherein the first duct extends to the bottom of thedrive cylinder.
 7. The method according to claim 1 wherein the inlet andoutlet include one way valves.
 8. The method according to claim 1wherein the first duct is in open communication with the tubing suchthat the hydraulic fluid is free to transfer back and forth between thetubing and the first side of the drive cylinder and wherein the secondduct is in open communication with the well casing such that theproduction liquid is free to transfer back and forth between the wellcasing and the second side of the drive cylinder.
 9. A method of pumpingliquid from a production location of a down hole well to a surfacelocation, the method comprising: providing a hydraulic pump at thesurface location for generating a flow of hydraulic fluid underpressure; providing a drive conduit extending from the pump to theproduction location, the tubing being connected so as to carry thehydraulic fluid under pressure through the tubing to the productionlocation; providing a return conduit from the production location to thesurface for transport of production liquid; operating a control valve tocause application of pressure in the hydraulic fluid to be applied intothe fluid in the drive conduit and to cause the pressure to be released;providing at the production location a reciprocating piston pump havinga pump cylinder with a pump piston movable therein, an inlet forproduction liquid and an outlet for the production liquid communicatingwith the pump cylinder such that in inlet stroke of the pump pistoncauses a volume of the production liquid to be drawn from the inlet intothe cylinder and an outlet stroke of the pump piston causes the volumeto be expelled from the outlet; locating the reciprocating piston pumpat the production location; arranging the reciprocating piston pump suchthat the stroke of the pump piston carries the volume of liquid from theproduction location into the return conduit; the reciprocating pistonpump having a drive piston mounted in a drive cylinder and arranged suchthat a drive stroke of the drive piston in the drive cylinder acts tocause the pump stroke of the pump piston in the pump cylinder;connecting the drive conduit to the reciprocating piston pump andproviding a first duct in the reciprocating piston pump so as totransfer hydraulic fluid under pressure from the drive conduit to thedrive cylinder at a position thereon on a first side of the drive pistonsuch that pressure in the hydraulic fluid is communicated to the drivepiston to drive the drive piston along the drive cylinder in the drivestroke; providing a second duct in the reciprocating piston pumpconnecting the drive cylinder on a second side of the drive piston tothe return conduit so as to allow communication of production liquidinto and out of the drive cylinder on the second side of the drivepiston; arranging the pressure in the hydraulic fluid to drive the drivepiston through the drive stroke to overcome the pressure of theproduction liquid against the second side of the drive piston and toovercome the pressure of the production liquid against the pump pistonto carry the volume of liquid from the production location below thepacking to the return conduit; and causing the drive piston to movethrough a return stroke by operating the control valve to effectreleasing the pressure in the hydraulic fluid so that pressure in theproduction liquid in the return conduit applied to the piston on thesecond side the pressure in applies a force to the drive piston.
 10. Themethod according to claim 9 wherein the density of the hydraulic fluidis arranged to be less than that of the production liquid so that thecolumn of production liquid in the well casing has a weight which isgreater than that of the hydraulic fluid in the tubing.
 11. The methodaccording to claim 9 wherein pressure from the production liquid belowthe packing is applied to the pump piston from the inlet and added tothe pressure from the production liquid on the drive piston so as tocause the drive piston to move through the return stroke.
 12. The methodaccording to claim 9 wherein the first and second ducts are formed in acylindrical body of the reciprocating piston pump.
 13. The methodaccording to claim 9 wherein the drive cylinder is located in thereciprocating piston pump below the pump cylinder.
 14. The methodaccording to claim 9 wherein the first duct extends to the bottom of thedrive cylinder.
 15. The method according to claim 9 wherein the inletand outlet include one way valves.
 16. The method according to claim 9wherein the first duct is in open communication with the drive conduitsuch that the hydraulic fluid is free to transfer back and forth betweenthe drive conduit and the first side of the drive cylinder and whereinthe second duct is in open communication with the return conduit suchthat the production liquid is free to transfer back and forth betweenthe return conduit and the second side of the drive cylinder.
 17. Areciprocating piston pump for pumping liquid from a production locationof a down hole well to a surface location, where the well includes awell casing and a tubing inside the well casing both extending from thesurface location to the production location; the reciprocating pistonpump comprising: a pump body having defined therein a pump cylinder anda drive cylinder; a pump piston movable in the pump cylinder; a drivepiston movable in the drive cylinder and arranged to drive the pumppiston such that a drive stroke of the drive piston in the drivecylinder acts to cause the pump stroke of the pump piston in the pumpcylinder; an inlet for production liquid at and an outlet for theproduction liquid communicating with the pump cylinder such that ininlet stroke of the pump piston causes a volume of the production liquidto be drawn from the inlet into the cylinder and an outlet stroke of thepump piston causes the volume to be expelled from the outlet; thereciprocating piston pump being arranged to be mounted in the wellcasing at the production location with a packing between thereciprocating piston pump and the well casing such that the inlet islocated below the packing and the outlet is located above the packingsuch that the stroke of the pump piston carries the volume of liquidfrom the production location below the packing to the well casing abovethe packing; a coupling for connecting the tubing to the reciprocatingpiston pump; a first duct in the pump body so as to transfer hydraulicfluid under pressure from the tubing to the drive cylinder at a positionthereon on a first side of the drive piston such that pressure in thehydraulic fluid is communicated to the drive piston to drive the drivepiston along the drive cylinder in the drive stroke; and a second ductin the pump body connecting the drive cylinder on a second side of thedrive piston to the well casing above the packing so as to allowcommunication of production liquid into and out of the drive cylinder onthe second side of the drive piston.
 18. The reciprocating piston pumpaccording to claim 17 wherein the density of the hydraulic fluid isarranged to be less than that of the production liquid so that thecolumn of production liquid in the well casing has a weight which isgreater than that of the hydraulic fluid in the tubing.
 19. Thereciprocating piston pump according to claim 17 wherein the drivecylinder is located in the reciprocating piston pump below the pumpcylinder.
 20. The reciprocating piston pump according to claim 17wherein the first duct extends to the bottom of the drive cylinder. 21.The reciprocating piston pump according to claim 17 wherein the inletand outlet include one way valves.
 22. The reciprocating piston pumpaccording to claim 17 wherein the first duct is in open communicationwith the tubing such that the hydraulic fluid is free to transfer backand forth between the tubing and the first side of the drive cylinderand wherein the second duct is in open communication with the wellcasing such that the production liquid is free to transfer back andforth between the well casing and the second side of the drive cylinder.